Production of aluminum powder



PRODUCTION F ALUIVIINUM POWDER Rudolf F. A. Wolf, Hamburg, Germany, assignor to Norddeutsche Afiinerie, Hamburg, Germany No Drawing. Application February 26, 1957 Serial No. 642,380

Claims priority, application Germany March 1, 1956 3 Claims. (Cl. 75-5) The present invention relates to an improved process for the production of aluminum powder which is par ticularly suited for the production of sintered bodies of high strength and hardness even at higher temperature ranges.

In order for aluminum powders to be suitable for the production of such sintered bodies they must possess the following qualities: an oxide content of about 612%, which is usually provided by oxidation of the aluminum particles, and is present in finely divided form; a poured weight of at least 0.7 g./cm. and a lubricant content below 0.3%.

Aluminum powders of this type, having the abovementioned qualities, are already known and they have previously been prepared by treating aluminum foil scraps, aluminum granules and the like in hammer mills and/or ball mills in a plurality of steps until the desired degree of fineness is achieved. As the comminution progresses, such processes must be carried out under protective gases having a lowered oxygen content so as to avoid danger of explosions on one hand and on the other hand provide for the desired oxidation. Towards the end of the processes, the grinding is effected with only a small quantity of lubricant in order that a partial welding of the particles is obtained which provides the desired higher poured weight of the resulting powder.

The mode of production employed produces a marked lamellar particle structure. The desired aluminum oxide content in the aluminum powder was partly achieved by surface oxidation of the new surfaces on the aluminum particles produced during the grinding operations and partly by a similar oxidation of the intermediate ground products in air during long periods of storage which must be interposed between the grinding operations and which can be of the order of weeks or even months.

The procedure described not only is cumbersome and time consuming, but also requires large expenditures for apparatus and energy and furthermore is subject to the danger of explosions.

It is the object of the invention to provide an improved process for the production of an aluminum powder of the indicated qualities which avoids the aforementioned disadvantages and furthermore provides considerable advantage.

According to the process of the invention the comminution is not effected by hammering or grinding of solid aluminum, but rather by comminution of molten aluminum to the required final extreme degree of fineness by spraying the melt. Furthermore, the oxide formation according to the invention is obtained by treatment of the aluminum powder obtained by the spraying operaates Patent 2,930,686 Patented Mar. 29, 1960 ice tion with hot water, rather than by the limited oxidation obtainable during the grinding procedure and intermediate storage periods. The hot water oxidation not only is extraordinarily rapid, but is easily controlled to produce the desired and predetermined degree of oxidation. The aluminum powder which has thus already been extensively oxidized is only then subjected to an aftertreatment in a ball mill. As the aluminum powder is already substantially in the desired state of comminution and oxidation, the time and energy required for the treatment in the ball mill are relatively slight and there is no longer any danger of explosion.

The preparation of the aluminum powder employed in the process according to the invention is preferably carried out by atomizing molten aluminum and quenching the atomized aluminum with water by the process describedin German application N 867 Ib/49 1. According to such process, the molten aluminum is atomized into a closed chamber by permitting a stream of molten aluminum to fall free from an outlet in a crucible and projecting an annular stream of gases or vapors employed as atomizing medium against such falling stream of aluminum. Preferably, the atomizing medium is blown against the falling aluminum stream in the form of a cone and at, at least, ultrasonic speed, care being taken that the point where atomization occurs is spaced suf- -ficiently from the outlet for the atomizing medium, such as an annular slit, so that the metal particles are not thrown back against the mouth of the atomizing nozzle by the eddies produced. This can, for example, be achieved by having the outlet for the metal stream project sufficiently beyond the annular outlet for the atomizing medium. A single annular nozzle or a plurality of nozzles concentric with the outlet for the molten aluminum stream can be employed to blow the atomizing medium against the molten aluminum stream. Steam at 3 to 15, preferably 8 to 1 2, atmospheres gauge pressure is expediently used as the atomizing medium. Water is sprayed from all sides into the atomizing chamber to condense the steam employed as the atomizing medium. For example, in atomizing 300 kg. of aluminum per hour, :1200 kg. of steam at 10 atmospheres gauge pressure and 30 m. of water are required per hour. The temperature of the water and aluminum powder mixture withdrawn from the atomizing chamber is controlled depending upon the composition of the aluminum to below C. and preferably to about 25 to 50 C. If the temperature of such mixture is too high, lively hydrogen evolution occurs. The higher the magnesium content in the aluminum powder produced, the lower the water temperature must be to prevent evolution of hydrogen.

The aqueous aluminum powder slurry leaving the atomizing apparatus is pressed out in normal filter apparatus, particularly filter presses with an end pressure of 5 atmospheres gauge pressure to reduce the moisture content of the powder to about 9% H O. The resulting crumbly filter cake is then pressed through a sieve with an 8 mm. mesh width and dropped on flat drying pans to a 30 mm. height. The pans are then placed in a drying cupboard, preferably with circulating air, and dried first at 30 C., then slowly raised to 40 to 45 C and finally to 60 C.

Previously in technical usage all contact of finely divided aluminum powder with water was avoided. However, according to the invention it was found that aluminum powder, produced by atomizing molten aluminum and quenching the atomized aluminum with water, reacts with water at elevated temperature with the formation of an oxide coating on the individual particles and that the quantity of oxide formed can be controlled, as it depends upon the water temperature and the duration of the treatment. The oxidation can, for example, be interrupted simply by adding cold water when the desired degree of oxidation has been reached.

The oxidation of the aluminum powder with Warm water can be eit'ected upon the dried product produced in the manner described above, but preferably such oxidation can beefi'ected in the aqueous slurry produced by the atomization procedure either by adjusting the quantity of cooling water supplied to the atomizing chamber to provide the necessary temperature for the oxidation, the oxidation being controlled by controllingthe time the aluminum powder remains in contact with the water at temperatures causing oxidation or by carrying out the atomization under conditions wherein the aluminum powder slurry produced is at a temperature lower than the temperature required for the oxidation and subsequently heating such slurry to the temperature for the period required for the oxidation.

The powder thus obtained by the atomization and oxidation despite its high oxide content, nevertheless, still isnot suited for the production of sintered products of high strength and hardness at elevated temperatures, as the oxide only lies upon the surface of the individual particles as a relatively thick coating and is not dispersely distributed. According to the invention it was found that such atomized and oxidized powder can be rendered suitable for the production of sintered bodies of the type described by giving it an aftertreatment in a ball mill or similarly operating apparatus with the addition of a small quantity of lubricant. This aftertreatment to a great extent causes the oxide to be kneaded into the metallic base of the particles andprovides fine distribution between the oxide and metal which is recognized as necessary in an aluminum powder for the production of sintered bodies of the type described.

The milling treatment, both in its purpose as well as its manner of being carried out, is substantially diiferent from the grinding processes employed by the prior art in the production of aluminum powder of the type described. lt is only of short duration and essentially re: quires no energy for comminution. Furthermore, it does not necessarily serve for a further oxidation. It also does not need to be interrupted by long storage periods and is not subject to dangers of explosion. The treatment of the oxidized aluminum powder according to the invention in a ball mill is completely safe as the oxide coating already present protects the metal from ignition and explosion. Consequently, no protective gases or other protective measures are required.

Example 300 kg. per hour of molten aluminum were atomized in a chamber with the aid of 1200 kg. per hour of steam at atmospheres gauge pressure as atomizing medium and 30 In. per hour of cooling water were sprayed into the chamber to produce an aqueous aluminum powder slurry having a temperature of about 40 C. 80% of the aluminum powder in such slurry had a. particle size in the range of 1-40 Such powder in the dry state had a poured weight of about 1 g./cm.

The slurry was then heated to a temperature of 85 C. for 3 minutes whereby an 8% oxide content'was achieved in the aluminum powder. The oxidation was terminated by cooling the slurry to below 40 C. by

addition of cold water. The resulting oxidized powder was then filtered oil and carefully dried at low temperatures so that practically all water was removed below a temperature of 40 C.

The oxide content of the treated aluminum powder can easily be controlled by selecting a proper temperature and duration of the oxidizing treatment. Oxidation begins upon heating the slurry over 50 C. and, as indicated, it can be terminated by cooling below 40 C. For example, a 25% oxide content can be achieved by heating the slurry at 85% C. for 10 minutes. Preferably, for the purposes of the invention, the oxide content of the final aluminum powder is between about 6% and 12%.

The resulting oxidized aluminum power in which the individual aluminum particles were coated with a relatively thick oxideskin was mixed with 0.5% stearic acid (lubricant) and treated for 5 to 6 hours in a ball mill under exclusion of air. The quantity of stearic acid remaining in the milled product was below 0.3% in View of reactions and evaporation. When the treatment of the oxidized aluminum powder is carried out under access of air, its oxide content increases by about 23%. This increase in oxide content must be taken into account when i a final desired oxide content is to be obtained by effecting a lower degree of oxidation during the preceding oxida tion in heated water.

The aluminum powder obtained after the treatment in the ball mill is admirably suited for the production of sintered bodies of high strength and hardness even at higher temperatures and possesses all of the qualities indicated as necessary for such a powder, namely, in having an'oxide content between 6% and 12% and over and a poured weight of between 0.7 and 0.8 g./cm. and being practically fat free.

Sintered bodies produced from such powder had the following characteristic values:

Tensile strength at room temperature 32-35 Inga/mm. Elongation at room temperature 68%. Hardness at room temperature About kg./mm. Brinell. Hardness at 250 C About 50 l g./mm. Brinell. Hardness at 350 C. About 35 kg./mm. Brinell.

I claim:

1. In a process for the production of an aluminum powder having an aluminum oxide content of at least 6% rendering it particularly suited for the production of sintered bodies of high tensile strength and hardness even at elevated temperatures, said powder having a poured weight of about 0.7-0.8 g./cc. and over and a lubricant content of less than 0.3%, the steps which comprise preparing an admixture of water with an aluminum powder obtained by the atomization of molten aluminum, subjecting said admixture to an elevated temperature suiticient to effect surface oxidation of the aluminum powder, cooling said admixture to a temperature below that at which such surface oxidation occurs after the oxide content of the aluminum powder has reached at least 6% to terminate such oxidation, drying the oxidized powder, admixing the dried powder with a relatively small quantity oflubricant and treating such admixture in a ball mill to knead the oxide into the metallic base of the particles to obtain an aluminum oxide containing aluminum powder having the required fine distribution between aluminum metal and oxide to render it suited for the production of sintered bodies of high tensile strength and hardness even at higher temperatures the quantity of lubricant admixed with the dried oxidized powder being such that after the treatment in the ball mill the milled powder has a lubricant content less than 0.3%.

2. The process of claim 1 in which the aluminum oxide content of said aluminum powder is between 6% and 12%.

References Cited in the file of this patent UNITED STATES PATENTS 2,671,995 Egan Mar. 16, 1954 2,671,996 Schneider Mar. 16, 1954 2,671,997 Egan et al Mar. 16 1954 2,678,880 Nuesch et a1. May 18, 1954 2,784,073 Michalke Mar. 5, 1957 FOREIGN PATENTS 553,672 Great Britain Oct. 8, 1942 697,445 Great Britain Sept. 23, 1953 700,694 Great Britain Dec. 9, 1953 OTHER REFERENCES British Intelligence Objectives Sub-Committee Final Report No. 729, Item No. 21, 1946, pages 121-130. Published by the British Intelligence Objectives Sub- Committee, London, England. 

1. IN A PROCESS FOR THE PRODUCTION OF AN ALUMINUM POWDER HAVING AN ALUMINUM OXIDE CONTENT OF AT LEAST 6% RENDERING IT PARTICULARLY SUITED FOR THE PRODUCTION OF SINTERED BODIES OF HIGH TENSILE STRENGTH AND HARDNESS EVEN AT ELEVATED TEMPERATURES, SAID POWDER HAVING A POURED WEIGHT OF ABOUT 0.7-0.8 G./CC. AND OVER AND A LUBRICANT CONTENT OF LESS THAN 0.3%, THE STEPS WHICH COMPRISE PREPARING AN ADMIXTURE OF WATER WITH AN ALUMINUM POWDER OBTAINED BY THE ATOMIZATION OF MOLTEN ALUMINUM, SUBJECTING SAID ADMIXTURE TO AN ELEVATED TEMPERATURE SUFFICIENT TO EFECT SURFACE OXIDATION OF THE ALUMINUM POWDER, COOLING SAID ADMIXTURE TO A TEMPERATURE BELOW THAT AT WHICH SUCH SURFACE OXIDATION OCCURS AFTER THE OXIDE CONTENT OF THE ALUMINUM POWDER HAS REACHED AT LEAST 6% TO TERMINATE SUCH OXIDATION, DRYING THE OXIDIZED POWDER, ADMIXING THE DRIED POWDER WITH A RELATIVELY SMALL QUANTITY OF LUBRICANT AND TREATING SUCH ADMIXTURE IN A BALL MILL TO KNEAD THE OXIDE INTO THE METALLIC BASE OF THE PARTICLES TO OBTAIN AN ALUMINUM OXIDE CONTAINING ALUMINUM POWDER HAVING THE REQUIRED FINE DISTRIBUTION BETWEEN ALUMINUM METAL AND OXIDE TO RENDER IT SUITED FOR THE PRODUCTION OF SINTERED BODIES OF HIGH TENSILE STRENGTH AND HARDNESS EVEN AT HIGHER TEMPERATURES THE QUANTITY OF LUBRICANT ADMIXED WITH THE DRIED OXIDIZED POWDER BEING SUCH THAT AFTER THE TREATMENT IN THE BALL MILL THE MILLED POWDER HAS A LUBRICANT CONTENT LESS THAN 0.3%. 